Entanglement between a Diamond Spin Qubit and a Photonic Time-Bin Qubit at Telecom Wavelength

Abstract

We report on the realization and verification of quantum entanglement between an NV electron spin qubit and a telecom-band photonic qubit. First we generate entanglement between the spin qubit and a 637 nm photonic time-bin qubit, followed by photonic quantum frequency conversion that transfers the entanglement to a 1588 nm photon. We characterize the resulting state by correlation measurements in different bases and find a lower bound to the Bell state fidelity of ≥ 0.77 ± 0.03. This result presents an important step towards extending quantum networks via optical fiber infrastructure.Quantum networks connecting and entangling longlived qubits via photonic channels [1] may enable new experiments in quantum science as well as a range of applications such as secure information exchange between multiple nodes, distributed quantum computing, clock synchronization, and quantum sensor networks [2][3][4][5][6][7][8][9][10]. A key building block for long-distance entanglement distribution via optical fibers is the generation of entanglement between a long-lived qubit and a photonic telecomwavelength qubit (see Fig. 1a). Such building blocks are now actively explored for various qubit platforms [11][12][13][14][15][16].The NV center in diamond is a promising candidate to act as a node in such quantum networks thanks to a combination of long spin coherence and spin-selective optical transitions that allow for high fidelity initialization and single-shot read out [17]. Moreover, memory qubits are provided in the form of surrounding carbon-13 nuclear spins. These have been employed for demonstrations of quantum error correction [18][19][20] and entanglement distillation [21]. Heralded entanglement between separate NV center spin qubits has been achieved by generating spin-photon entangled states followed by a joint measurement on the photons [22].Extending such entanglement distribution over long distances is severely hindered by photon loss in the fibers. The wavelength at which the NV center emits resonant photons, the so-called zero-phonon-line (ZPL) at 637 nm, exhibits high attenuation in optical glass fibers. Quantum-coherent frequency conversion to the telecom band can mitigate these losses by roughly 7 orders of magnitude for a distance of 10 km [23,24] and would enable the quantum network to optimally benefit from the existing telecom fiber infrastructure. Recently, we have realized the conversion of 637 nm NV photons to * These two authors contributed equally to this work. † R.Hanson@tudelft.nl 1588 nm (in the telecom L-band) using a difference frequency generation (DFG) process and shown that the intrinsic single-photon character is maintained during this process [25]. However, for entanglement distribution an additional critical requirement is that the quantum information encoded by the photon is preserved during the frequency conversion.Here we demonstrate entanglement between an NV center spin qubit and a time-bin encoded frequencyconverted photonic qubit at telecom wavelength. The concept of our experiment is d...

Designing a quantum network protocol

“…However, real deployments of quantum networks are around the corner with the first inter-city network scheduled to go online within the next few years [4]. Much essential work is being done to build quantum hardware to make this possible [8,58,75,83,92] and we are now entering a new phase of development where we need to learn how to build quantum communication systems. Work in this field has been slowly emerging over the last few years (see e.g.…”

“…Furthermore, this method overcomes transmission losses as long-range entanglement can be created by "stitching" shorter-range pairs together through a process called entanglement swapping [10] which means that it is not necessary to transmit qubits directly along the entire path. Entanglement generation between two directly connected nodes with a quantum memory has been demonstrated at distances of up to 1.3 km [41] and work is underway to build a three-node setup and extend the inter-node distances to several kilometres [28,83].…”

Making Diamond Qubits Talk to Light

“…The resulting entangled state is quantified by its concurrence C 0.42 (6) [C 0.79 7after correcting for readout errors] (Appendix E). The entanglement fidelity between a photonic qubit and the SiV center is competitive with that achievable in other systems [8,60,61], and is limited primarily by residual reflections from the cavity. This can be straightforwardly increased, allowing for recent demonstrations of high-fidelity (F 97%) spinphoton entangled states [39], which are a fundamental resource for quantum communication [2] and quantum computing schemes [5], and can be used, for example, to demonstrate heralded storage of a photonic qubit into memory [29].…”

Telecom photon interface of solid-state quantum nodes

“…The low emission rate into the ZPL limits the rate of entanglement generation. The emission fraction into the ZPL could be enhanced by a microcavity via the Purcell effect [14], while a difference frequency generation, used in recent experiments, achieved conversion of single NV photons into telecom wavelength with 17% efficiency [15,16]. However, the signal-to-noise ratio was limited by pump-induced noise in the conversion process [17,18] and resonance driving at cryogenic temperature is required, preventing room temperature applications.…”

“…The conventional way to overcome this limit is to perform parametric down conversion to convert the photons into telecom-wavelength photons (tele-photons), as demonstrated in several systems including quantum dots [5][6][7], trapped ions [8-10] and atomic ensembles [11][12][13].The low emission rate into the ZPL limits the rate of entanglement generation. The emission fraction into the ZPL could be enhanced by a microcavity via the Purcell effect [14], while a difference frequency generation, used in recent experiments, achieved conversion of single NV photons into telecom wavelength with 17% efficiency [15,16]. However, the signal-to-noise ratio was limited by pump-induced noise in the conversion process [17,18] and resonance driving at cryogenic temperature is required, preventing room temperature applications.An alternative approach is to work with the microwave interface of NV centers and then up-convert the signal to the desired optical domain.…”